In the manufacture of small moldings, for which production of the relatively small quantities of reaction mixture is required, the throughflow time, (i.e., the time required to produce a certain amount of mixture from the reaction components) cannot be arbitrarily reduced in order that the components have sufficient mixing time. Attempts have therefore been made to make the nozzle bores of the injection nozzles as small as possible. The diameters of the spindles of the needle nozzles had to be adapted accordingly.
With decreasing spindle diameters, the difficulties involved in the manufacture of these spindles increase due to the problems which arise during the hardening process at the junction between the spindle and the nozzle stem because of the difference in their diameters. Even in the cases where nondistorting nozzle needles with spindles are used, the spindle is in danger of being permanently deformed or even broken by even the smallest of foreign bodies in or around the nozzle bore. The danger of damage is greater, the smaller the spindle diameter.
Although it has been possible to produce and use nozzle needles with a spindle down to 0.8 mm in diameter, the manufacturing difficulties have become so serious and the frequency of spindle breaks has become so great that the number of nozzle needles which have had to be rejected after production and the operational difficulties caused by spindle fractures and destruction of the nozzle bores have become unacceptable. The object of the present invention is to structurally improve the injection nozzles used in apparatus of the above-mentioned type to such an extent that the nozzle needles can be produced without an unacceptable level of rejects and that the spindles show high resistance to breakage, even when very small in diameter.